Method of adding rare earth metals or their alloys into liquid steel

ABSTRACT

When one or more rare earth metals or their alloys are added into liquid steel in an intermediate vessel disposed between a ladle and a mold, the resulting steel exhibits excellent impact property.

United States Patent [191 Nemoto et al.

[ Mar. 18, 1975 METHOD OF ADDING RARE EARTH METALS OR THEIR ALLOYS INTOLIQUID I STEEL [75] Inventors: Hidetaro Nemoto; Takaho Kawawa,

both of Tokyo; I-Iideki Sato, Yokohama; Eiichi Sakamoto, Kamakura;Takayuki Koyano, Tokyo; Takanori Anzai, Yokohama,

l ftlav [73] Assignee: Nippon Kokan Kabushiki Kaisha,

Tokyo, Japan [22] Filed: May 1, 1973 21 Appl. No.: 356,087

[52] U.S. Cl. 75/58, 75/53 [51] Int. Cl. C216 7/00 [58] Field ofSearch... 75/58, 53, 57, 123 E, 126 G,

[56} References Cited UNITED-STATES PATENTS 2,683,661 7/1954 Tisdale etal. 75/58 OTHER PUBLICATIONS Schwartzbart, H. et 21]., Rare Earthslmprove Impact Properties in Iron Age, May 1955, pp. 103-106.

Primary Examiner-Walter R. Satterfield Attorney, Agent, or Firm-MoonrayKojima [57] ABSTRACT When one or more rare earth metals or their alloysare added into liquid steel in an intermediate vessel disposed betweena-ladle and a mold, the resulting steel exhibits excellent impactproperty.

3 Claims, 4 Drawing Figures PATENTEU NARI 81975 sum 1 nr g Elk 5 O. 5 w8 9. O A... m. W K K K K .n v 8. -0 O E K -4. 1 O H K 12 O O C S C v O 2O 8. 6 4. 2 O 8. 6 4. 2 2 2 I l l I. I O O O O 00 cozE mocoo E wucozotcmucou m BACKGROUND OF THE INVENTION This invention relates to anovel method of desulphurization to prevent center segregation ofsulphur in casting of steel and more particularly to a method of addingcontinuously or otherwise rare earth metals or their alloys asdesuphurization agents into liquid steel.

It is well known in the art that one effective means of improvingmechanical properties of steel, particularly, impact strength, is tolower the sulphur content of steel. Various proposals for lower sulphurcontent in 'steel have been made and practiced besides desuphurizationduring steel refining, such as:

1. Adding desulphurization agent during blowing in of such gases as Nand Ar.

2. Stirring in molten steel in a rotary kiln type vessel,

with desulphurization agent.

3. Utilizing pumping action caused by rotating means in a hot metalladle. 4. To revolve slag-into eddy current caused by the rotatingmeans. I

5. To give an eccentric motion to the vessel to cause hot metal swingmovement and react same with desulphurization agent.

6. To utilize degassing facilities for desulphurization purposes.

As desulphurization agents in these processes, c'alcium, carbide, sodaash, lime, metallic magnesium, etc. are used, either singly or incombination of two or more. However, various experiments performed bythe inventors hereof revealed that low sulphur steel obtained throughthe above mentioned prior processes were defective in precipitation andshape of the sulfide. As is clearly understood from FIG. I, explainedhereinbelow in more detail, so long as S remains in the molten steel asa solute, its concentration gradually heightens as its solidificationproceeds and precipitates in the form of MnS among dendrites and becomesconcentrated segregation in the remaining molten steel. As is known inthe art, the concentration of solute elements is obtained from thefollowing formula: C CK (lg)" wherein C= cencentration at thesolidification rate 3; Co concentration at an early stage; K effectivedistribution coefficient and g solidification ratio (O-l It is axiomaticand well known that this phenomenon is unavoidable from the fact thatsolubility of S in solidified stee is extremely small. Because of thisextremely low sulfur content, even if S concentration on the whole isquite low, segregation of sulfide concentrated correspondingly which isoften seen in high sulphur steel still aparently exists in the centerpart thereof. When such a slab is rolled and made into a product, itsimpact property, especially in the transverse direction, is unavoidablyinferior. The MnS takes a comparatively round shape or of an eutecticcrystal shape at the time of casting, but it is easily subjected toplastic formation during rolling. Thus, the resulting-product iselongated excessively in the rolling direction. Such phenomenon, asshown in FIG. 2, as explained further hereinbelow, cannot be eliminatedsimply by lowering the S content in steel.

For improvement of impact strength of steel, it is necessary to take thefollowing points into consideration, besides merely lowering S contentin steel. First,

S in the molten steel should not be precipitated as MnS at the time ofsolidification. That is to say, S content in some way should be fixedprior to its precipitation as MnS. If this can be done, then itsconcentration will not occur and prevention of segregation at the finalstage of solidification will be facilitated.

Second, it is desirable to change sulfide into something which will notbe easily subjected to plastic deformation in the subsequent rollingstep. This will enable prevention of elongation of MnS and improvementof mechanical properties, particularly impact strength. If the variousproblems mentioned above were to be resolved, then it will not benecessary to provide separate desulphurization process, 'but in fact,could be conducted during ordinary casting operation. This willcontribute to improvement of productivity, lowering of manufacturingcosts, and obtain an improved product.

However, as is known to workers in the art, all prior efforts to solvethe above mentioned problems during the casting of liquid steel havebeen accompanied by extreme difficulties and have been quiteunsatisfactory. Those solutions heretofore proposed have not beeneffective nor suitable.

SUMMARY OF THE INVENTION This invention overcomes the aforementioneddefects and problems, and advantageously enables increased productivity,lowering of manufacturing costs and production of an improved product.As explained hereinabove in the context of the prior art, the presentinvention enables desulphurization during ordinary casting and henceeliminates the prior art separate process of desulphurization. Y

Briefly, the invention encompasses addition of rare earth metals ortheir alloys or mixtures thereof substantially into liquid steel at alocation between a ladle and a mold. The processis useful for continuouscasting or other types of casting. An object of the invention is toprovide a desulphurization method in which it is possible to fix S ofsolute in steel prior to precipitation of ordinary MnS, and thus toprevent S center segregation. Another object is to provide adesulphurization method which improves the impact properties in thetransverse direction of a rolled steel.

Other objects, features and advantages of the invention will become moreapparent from the following description and accompanying drawing.

BRIEF DESCRIPTION OF DRAWING FIG. 1 is a graph depicting the manner ofconcentration of solute element S;

FIG. 2 is a graph depicting the relationship between S content in steeland its impact value;

FIG. 3 depicts a diagrammatic view of an illustrative embodiment of theinvention; and

.FIG. 4 is a graph depicting changes of Charpy impact value compared toCe/S ratio.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The desulphurizing agentin the present invention desulphurizes and also plays an important partof fixing S content in steel before the S content is precipitated asMnS. The term rare earth metal is well known in the art. Any rare earthmetal and/or their alloys or mixtures thereof may be used in thisinvention. Examples thereof may be Lanthanum (La), Cerium (Ce),Presodimium (Pr), Neodymium (Nd) and others. Al-

loys of rare earth metals are often called misch metals.

Rare earth metals, as is known, have an extremely Y strong affinity foroxygen, and hence, require extraordinary care for their use. The degreeanytype of care required for heir use is different from those requiredfor ordinary and conventionally known desulphurizing agents such ascalcium. For example, it is possible to fill an iron pipe with the rareearth metal and fix the pipe within a casting mold, which may graduallybe dissolved by bottom blowing in the ordinary ingot making process.However, this is practically not feasible because of the strong affinityof rare earth metals with oxygen. Accordingly, optimum conditions forthe location where the rare earth metal is added, the shape or formthereof, the rate of addition and amount thereof should be providedforasdiscussed hereinbelow.

The rare earth metals, their alloys or mixtures thereof, may be addedinto liquid steel in a vessel which is disposed betweeen a ladle and amold. Ordinarily, the liquid steel is poured directly from the ladleinto the mold. We have discovered that an intermediate vessel should beprovided between the ladle and mold for addition of the rare earth metalto the liquid steel, in order to obtain highest yield of additives, toemploy the purest steel available and to easily control additiveamounts. If, for instance, the rare earth metals were to be added to theladle, they would unavoidably react with the slag in the ladle eitherbefore or after pouring. The yieldwould then be unavoidably lowered dueto oxidation. Undesirable reaction of the rare earth metals withrefractories in the ladle also'occur since the casting takesconsiderably long periods of time, regardless of whether ordinary orcontinuous casting is used. Thus, there is no merit in adding the rareearth metals to the ladle. When addition of the rare earth metals to thecasting mold was studied, an excessively undesirable influence to thesteel was confirmed to exist. This is because, it is thought, of theusual presence ofa covering agent on the bath surface which will reactwith the added rare earth metal, which in turn deteriorates the purityof the surface layer of the cast pieces. Even if oxidation of rare earthmetals was to be prevented, sulfide with rare earth metal tends togather toward the surface layer, and since such sulfides have a largerdensity (e.g. 6-7 g/cm) compared to other non-metallic inclusions, theirability to float and separate within the casting mold is quite inferior.This inferiority causes deterioration of cleanliness of the slabs andmaterials. This drawback is much too great to offset the utility ofraising yield of addition. There is no reason to recommend such anaddition to the casting mold.

Addition of rare earth metals (it is to be understood that use of suchterm herein means also their alloys and mixtures thereof) in accordancewith the present invention may be made to the intermediate vessel.Generally, concentration of C, P, and S in the final stage ofsolidification is seen due to the behaviour of solute elements.Concentrated segregation of S having small distribution coefficientappears in the center part and causes deterioration of the material asmentioned above. Thus, it is desirable to provide certain conditions foroptimal results, including the use of the intermediately located vessel.

Liquid steel to be poured to the vessel to which the the rare earthmetal is added, should be substantially completely deoxidized forpreferred effects. If there is any free oxygen remaining in the steel;for example Ce O C302 or C3203; La La added rare earth metal will tendto be wasted and lower the yield of desulphurization agent. For the samereason, rare earth metals to be added should be substantially completelysealed until they enter into the liquid molten steel. As will bedescribed later, this can be easily done, for example, by such means asan immersion type nozzle using inert gasses. Rare earth metal to beadded in this manner may preferably be in the form of a wire. Particle,granular or mass forms may be employed with desired effects. v The rareearth metal may be fed deeply into the liquid steel in the intermediatevessel under suitable conditions of shape, size and initial speed ofaddition. If the rare earth is in powder form, it has been found thatsuch is difficult to feed deeply into the liquid steel. Experimentsperformed by the inventors showed that the diameter of the metalparticles, or other forms. should preferably be at least 2 mm or more inorder to physically feed same into liquid steel with a suitable desiredspeed. A diameter of at least 2 mm or more is thus preferred as aminimum size. Any size above this may also be used. The size may beselected dependent upon the amount to be added continuouslycorresponding to the casting speed. Such size, amount and speed can bereadily calculated by the worker by observing such factors as rate offull utilization and entry into the liquid. A significant factor is theamount which is continuously added. The amount depends upon the 5content in the liquid steel to a large degree. However, a different typeof care is required in continuous casting operation where one charge ofmolten metal is continuously case and drawn as a slab or predetermineddimensions. The most suitable addition speed and hence amountcorresponds to the casting speed in order to provide sufficient rareearth metals for reaction with the S content. if the amount to be addedcontinuously was not suitable for the casting speed, then variousdefects, such as above mentioned become unavoidable. Such an amount maybe determined as just mentioned from the casting speed of the liquidsteel which corresponds to the drawing speed of slab in known continuouscasting processes. The invention is not to be construed to be limited tospeed, amount of addition, although such factors as size and speed arespecific features of the invention. The use of rare earth metals hasheretofore not been known in the process of this application.

FIG. 3 shows one illustrative embodiment by which the invention processmay be practiced. One feature of the invention lies in arrangement of anintermediate vessel 2 between ladle 1 and mold 3. First, tapped liquidsteel from refining furnace (not shown) is substantially completelydeoxidized by Al, Si and Mn in ladle 1. It is recommended that an inertgas such as N Ar and the like be blown through a nozzle (not shown) madeof porous bricks or the like provided at the bottom part of ladle l formaking clean molten steel and producing uniform temperature of thesteel. The liquid steel is then poured into an intermediate vessel, suchas a tundish 2. A teeming stream is prevent from airoxizing by anenclosing device .4. Covering 12 on the bath surface is used to preventoxidation of theliquid steel and to keep the temperature high. Immersiontype nozzle 13 is used for casting purposes. A covering agent 14 is alsoused on the bath surface of casting mold 3. Thus, the liquid steel whichis substantially completely deoxidized, made 'clean and of uniformtemperature, does not come into contact with the atmosphere again. Inthis manner, addition of rare earth metal brings about highest effects.The rare earth metals are also substantially prevent from coming intocontact with the atmosphere. 7

The rare earth metal or alloy thereof may be fed deeply into the liquidsteel by an arrangement such as shown in FIG. 3. This arrangementcomprises a hood 9, a feed ring 10 for inert gas feed, refractory pipe11 immersed deeply into the liquid steel in vessel 2. There is nosubstantial possibility of the added rare earth comprise a wire feeder5, a reel 6 for the wire 7 which is shown being uncoiled and guide pipe8 for the wire. The arrangement for adding the rare earth metal to theliquid steel may be similarly constructed for metals having the form ofparticles, granules, or massive. The liquid steel is thus desulpherizedefficiently without being subjected to air pollution and may hence becontinuously finished as a product. It will be readily understood fromthe above that there are no factors present which would reduce theproductivity.

The above combination, i.e. a ladle, an intermediate vessel and a mold,may be most efficiently and effectively put into practice for knowncontinuous casting processes. In such case, the intermediate vessel maybe a tundish of the process.

The following specific example of the present invention, which isapplied to continuous casting process, is set forth for illustrative andnot limiting purposes.

Manufacturing conditions:

Steel making furnace used: l00t LD converter. Deoxidation agent: Al, Si,Mn Composition of liquid steel (taken from teeming stream) 0.12% C 0.22%Si 0.69% Mn 0.024% P 0.012% S 0.038% Sol.Al remainder iron andunavoidable elements. Treatment in the ladle: Makin clean liquid metaland uniform temperature ac ieved by N gas blown through porous bricksprovided at bottom of ladle. Desulphurization process:

Processing Apparatus: as shown in FIG. 3. Rare earth metals used: mischmetal of Ce 45%,

La 30%, Nd 15% and others. Conditions for addition: wire of 4 mmdiameter is continuously added to the tundish at speed of 0.10 m/sec atthe rate of 0.03% per ton of liquid steel. Casting speed: Continuouscasting at rate of 1.24 t/min. Casting effects:

desulphurizing effect: The 0.012% S content is lowered to 0.008%. Nosegration in steel. lmpact value: FIG. 4 shows the values as compared tothat which has not been subjected to desulphurization with rare earthmetals.

From an analysis of the above factory scale experiment, it was foundthat there was disappearance of S segregation which was seen in thecenter part of the slab (such as by sulphur print, not shown in thedrawing) and that desulphurization had been performed in an excellentmanner. This further revealed that addition of misch metals, and rareearth metals, fixes C content in steel as sulfide ofthe added rare earthmetals and leaves no room for precipitation as MnS. Accordingly, thereis no concentration in the solidification stage, nor is theresegregation. Such a situation, as shown more clearly in FIG. 4, bringsabout improved impact properties.

FIG. 4 is a graph showing the results of the above factory scaleexperiment in which the Ce/S ratio was varied, and the resulting impactvalues. Specifically, the results of V notch Charpy test at 0C andperformed on a 9 mm thickness of plate are shown. According to thisgraph, there appears to be no significant difference in the rollingdirection caused by addition of said rare earthmetal, but surprisingly,the impact value in the transverse direction is remarkably improved, incontrast to prior art steels, to'an extent thatCe/S isat its maximumvalue of about 1.5 (1.2 to 2.0). Such remarkable improvement is caused,it is thought, by no other reason than the S content in the steel beingfixed as sultide with CeS, LaS or as compound sulfide thereof or asoxysulfide. Thus, there is no substantial elongation in the longitudinaldirection at the rolling stage, as would be the case for prior artsteels in which MnS is present. Hence, control of the shape wassuccessfully achieved. Sulfides of rare earth metals exceeded ourexpectations in eliminating substantially completely all of thedrawbacks caused by the sulfides of the conventional art, such as MnS.

In the above mentioned experiment, a wire of about 4 mm in diameter inan amount of about 0.03% per ton of liquid steel was added continuouslyat the speed of 0.10 m/sec. as against a casting speed of 1.24 t/min.This is one illustration of a continuous adding speed corresponding tothe casting speed as discussed before. Numerous variations of S contentin the ladle, diameter of wire and adding speed employed correspondingto one casting speed are foreseeable and the present invention isintended to cover all such variations.

According to another experimentby the inventors, it was confirmed that awire, such as above mentioned may be preferably effectively fed deeplyinto the liquid steel when the adding speed was 0.05 m/sec as above.Thus, the rare earth metals or misch metals or any combination thereofmay preferably be used in the present invention in the following rangeswhen used in the form of a wire: diameter: 2mm or above; adding speed:0.05 m/sec. or above.

The inventors then performed an ultrasonic flaw detection test forcleanliness of the resulting steel and confirmed that there was nodifference in the defective echos in the steel to which addition of rareearth metals had been made and in the steel to which no addition hadbeen made. This confirmed that there was no deterioration of cleanlinessof steel caused by the addition of rare earth metals or misch metals.

The present inventive process is also very effective in other castingprocesses. For example, the known bottom pouring process is one of theembodiments wherein the mold of the combination of ladle, intermediatevessel and mold may be replaced with a teeming pipe.

The foregoing description is intended to be illustrative of theprinciples of the invention. Numerous variations and modificationsthereof would be apparent to the worker skilled in the art. All suchvariations and modifications are to be considered to be within thespirit and scope of the invention.

What is claimed is:

1. In a continuous casting process using a succession of separate ladle,tundish and mold, the steps comprismg substantially completelydeoxidizing molten steel in said ladle by addition of aluminum, silicon,manganese or any combination thereof;

thereafter pouring said molten steel into said separate tundish havingmeans for preventing oxidation of said steel therein;

adding a rare earth metal selected from the group consisting of La, Ce,Pr and-Nd, any alloy thereof or any combination thereof, in the ratio ofrare earth metal to sulfur of between 1.2 and 2.0 and in the form of awire of a diameter of at least 2.0 to

4.0 mm, deeply into said substantially completely deoxidized steelcontained in said tundish, without being exposed to oxidation, saidadding being at the rate of at least 0.05 m/sec; and

thereafter passing said molten steel to said separate mold having meansfor preventing oxidation thereof, thereby to produce a steel havingimproved impact strength.

2. Process of claim 1, wherein said wire is added at the rate of 0.03%per ton of said molten steel and at the speed of addition of 0.l0 m/secwith the rate of casting being at about 1.24 t/min.

3. Process of claim 1, wherein cerium is used and the cerium/sulfurratio is 1.5.

1. IN A CONTINUOUS CASTING PROCESS USING A SUCCESSION OF SEPARATE LADLE,TUNDISH AND MOLD, THE STEPS COMPRISING SUBSTANTIALLY COMPLETELYDEOXIDIZING MOLTEN STEEL IN SAID LADLE BY ADDITION OF ALUMINUM, SILICON,MANGANESE OR ANY COMBINATION THEREOF; THEREAFTER POURING SAID MOLTENSTEEL INTO SAID SEPARATE TUNDISH HAVING MEANS FOR PREVENTING OXIDATIONOF SAID STEEL THEREIN; ADDING A RARE EARTH METAL SELECTED FROM THE GROUPCONSISTING OF LA, CE, PR AND ND, ANY ALLOY THEREOF OR ANY COMBINATIONTHEREOF, IN THE RATIO OF RARE EARTH METAL TO SULFUR OT BETWEEN 1.2 AND2.0 AND IN THE FORM OF A WIRE OF A DIAMETER OF AT LEAST 2.0 TO 4.0 MM,DEEPLY INTO SAID SUBSTANTIALLY COMPLETELY DEOXIDIZED STEEL CONTAINED INSAID TUNDISH, WITHOUT BEING EXPOSED TO OXIDATION, SAID ADDING BEING ATTHE RATE OF AT LEAST 0.05 M/SEC,AND THEREAFTER PASSING SAID MOLTEN STEELTO SAID SEPARATE MOLD HAVING MEANS FOR PREVENTING OXIDATION THEREOF,THEREBY TO PRODUCE A STEEL HAVING IMPROVED IMPACT STRENGTH.
 2. Processof claim 1, wherein said wire is added at the rate of 0.03% per ton ofsaid molten steel and at the speed of addition of 0.10 m/sec with therate of casting being at about 1.24 t/min.
 3. Process of claim 1,wherein cerium is used and the cerium/sulfur ratio is 1.5.